Overload relays are electrical switches typically employed in industrial settings to protect electrical equipment from damage due to overheating in turn caused by excessive current flow. In a typical case, the electrical equipment is a three phase motor which is connected to a power source through another relay commonly referred to as a contactor. A typical contactor is a heavy duty relay having three switched power paths for making and breaking each of the circuits connected to the three phase power source. The motion required to make and break the contacts is provided magnetically as the result of power flow through a coil which in turn is energized by current whose flow is controlled by another switch, typically remotely located.
In a conventional set up, an overload relay is connected in series with the control switch for the coil of the contactor. When an overload condition is detected by the overload relay, the same cuts off power to the coil of the contactor, allowing the contactor to open and disconnect the electrical equipment that is controlled by the contactor from the source of power to prevent injury to the electrical equipment.
In the past, overload relays have utilized resistive heaters for each phase which are in heat transfer relation with a bimetallic element which in turn controls a switch. When an overload is sensed, as, for example, when there is sufficient heat input from the resistive heater to the bimetallic element, the bimetallic element opens its associated switch to de-energize the contactor coil and disconnect the associated piece of electrical equipment from the source of power.
More recently, the resistive heater-bimetallic element type of relay has been supplanted by electronic overload relays. See, for example, commonly assigned U.S. Pat. No. 5,179,495 issued Jan. 12, 1993, to Zuzuly, the entire disclosure of which is herein incorporated by reference. Outputs of such circuitry typically are relatively low powered and as a consequence, in order for the output to control the contactor coil current, a solid state switch may be required.
In one case, an overload relay, once tripped, will remain in an open position, preventing the flow of current to the contactor, and must be manually reset. Usually, a push button is employed so that the person operating the equipment may push the push button to cause a reset of the system, closing the contacts of the overload relay to again allow current to flow to the contactor coil which in turn will close the contactor contact and provide current to the electrical equipment.
At the same time, applicable standards require that the construction of the push button and associated mechanical components be such that the overload relay contacts may open in the event of an overload even when the push button has been pushed for reset purposes. While this will prevent damage to the electrical equipment if an overload condition occurs or continues during the process of resetting the overload relay, the purpose of the rule is to require that the overload relay construction be such that it cannot be defeated by holding down or jamming the push button in the reset position. An overload relay having such a feature is known as a "trip free" overload relay.
In some instances, it is also desirable to provide a means whereby an overload relay will automatically reset, assuming that the overload condition that tripped it in the first place has been alleviated in the meantime. In such cases, the trip mechanism will periodically receive a reset signal from the control circuitry and the mechanical construction should be such that resetting will occur automatically without manipulation of a reset push button or the like.
It is also desirable that an overload relay be provided with means whereby the relay condition may be switched manually for test purposes. Thus, the overload relay should be capable of being reset or tripped manually without manipulating a reset push button or actually encountering an overload.
In many instances, it is also desirable that the overload relay be provided with a means that may be utilized to momentarily interrupt flow of power to the piece of electrical equipment being monitored by the overload relay.
The present invention is directed to providing an overload relay having the foregoing capabilities and features along with others in a reliable, mechanical trip mechanism that can be economically manufactured.